Transformer having an electrically symmetrical tapped winding

ABSTRACT

A transformer having a winding which may be tapped without axially moving the electrical center of the winding. The winding includes at least four coil sections, with two of the sections having the same number of turns and the other two sections each having three times that number of turns. The coil sections are arranged in axially adjacent pairs with the lower end of the smaller section directly connected to the lower end of the larger section in a pair of coil sections. Tap leads are connected to ends of the coil sections and across one-third of the turns in the larger coil sections. Tap changing is accomplished by interconnecting the tap leads to tap out equal portions of the winding on both sides of the electrical center line.

[45] Aug. 27, 1974 Primary Examiner-Thomas J. Kozma Attorney, Agent, orFirm-.1. R. Hanway ABSTRACT A transformer having a winding which may betapped without axially moving the electrical center of the winding. Thewinding includes at least four coil sections, with two of the sectionshaving the same number of turns and the other two sections each havingthree times that number of turns. The coil sections are arranged inaxially adjacent pairs with the lower end of the smaller sectiondirectly connected to the lower end of the larger section in a pair ofcoil sections. Tap leads are connected to ends of the coil sections andacross one-third of the turns in the larger coil sections. Tap changingis accomplished by interconnecting the tap leads to tap out equalportions of the winding on both sides of the electrical center line.

7 Claims, 6 Drawing Figures @000 5& an?) 7 ELECTRICALLY SYMMETRICALTAPPED WINDING [75] Inventor: Ram R. P. Sinha, Athens, Ga.

Pittsburgh, Pa.

Filed: Aug. 20, 1973 Appl. No.: 389,611

us. 336/150; 323/435 [51] Int. H01f 21/12 Field of 336/l5 0;'323/43.5

References Cited UNITED STATES PATENTS 4/1967 Russell......... 336/150 XUnited States Patent Sinha TRANSFORMER HAVING AN [73] Assignee:Westinghouse Electric Corporation,

8 ,ooo ooocooo 000 9 4 A 9 QOOOOOOOO OOOOOOO BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates, in general,to electrical inductive apparatus and, more specifically, totransformers having a tapped winding.

2. Description of the Prior Art Coaxially positioned high-voltage andlow-voltage windings are used in many power transformers. It isdesirable to construct the windings in such a manner that theirelectrical centers will be at substantially the same axial position. Ifthe electrical centers are not so aligned, forces acting between thewindings when under high stress conditions, such as during ashortcircuit condition, tend to make the windings telescope with respectto each other.

In non-tapped windings, it is sometimes difiicult to place theelectrical centers at the proper position due to the manufacturingvariations. In tapped windings, the winding arrangement is such that,during. certain positions of the tap changer, the electrical center ofthe tapped winding is moved from the desired axial electrical center.This occurs since unequal positions of the tapped winding are energizedon each side of the desired electrical center position.

Constructing tapped transformers which exhibit unbalanced electricalcenters in certain tap positionsinvolves the necessity of providingadditional winding braces to prevent telescoping of the winding.Therefore, it is desirable, and it is an object of this invention, toprovide a transformer having a tapped winding which does notsubstantially change its electrical center in different tap positions.

SUMMARY OF THE INVENTION There is disclosed herein a new and usefultransformer having a winding arrangement which provides a tapped windingthat maintains its axial electrical cen ter throughout the tappingrange. The tapped winding includes at least four coil sections to whichtap leads are connected. The individual coil sections of each pair ofcoil sections are located at the same radial position as the othersection in the pair. The two pairs of coil sections are locatedcoaxially with respect to each other. One of the coil sections in eachpair has three times the number of turns as its axially adjacent coilsection in the pair and is tapped across one-third of its turns. Thelower ends of each coil section in the pair are substantiallynon-inductively connected to each other. Tap leads from the coilsections are connected to a tap changerwhich connects togetherpredetermined pairs of the tap leads to provide the desired windingrating. In each tap position, an equal amount of the coil sections ineach pair is tapped out above and below the center of the winding tomaintain electrical balance in the tapped winding.

BRIEF DESCRIPTION OF THE DRAWING Further advantages and uses of thisinvention will be-,

come more apparent when considered in view of the following detaileddescription and drawing, in which:

FIG. lis a view of a transformer having a tapped winding;

FIG. 2 is a schematic view of a tapped winding con-' structed accordingto the prior art;

FIG. 3 is a force diagram'for a tapped winding constructed asillustrated in FIG. 2;

FIG. 4 is a schematic view of a tapped winding, constructed according tothis invention and taken generally along the lines IV-IV of FIG. 1;

FIG. 5 is a force diagram for a tapped winding constructed asillustrated in FIG. 4; and

FIG. 6 is a table which illustrates the electrical symmetry of thewinding shown in FIG. 4 when in different tap positions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Throughout the followingdescription, similar reference characters refer to similar elements ormembers in all of the figures of the drawing.

Referring now to the drawing, and to FIG. 1 in particular, there isshown a transformer assembly having a wound magnetic core 10 mountedwithin the frame 12. A winding structure 14 is positioned throughwindows in the two sections of the magnetic core 10. The low voltageleads 16 and the high-voltage leads 18 extend from the winding structure14 for connection to the appropriate associated apparatus or cables. Thetap leads 20 extend from the winding structure 14 and connect to theno-load tap changer 22 which is mounted to the frame member 24. The tapchanger 22 may be set in different positions to change the rating of thehighvoltage winding. The tap changer illustrated in this specificembodiment is capable of providing five different high-voltage windingratings and is described, in detail, in US. Pat. No. 3,467,794, which isassigned to the assignee of this invention.

The high-voltage winding is wound coaxially'with respect to thelow-voltage winding and comprises a plurality of conductor layers eachhaving one or more coil sections which have a plurality of conductorturns. FIG. 2 represents, in a partially'schematic view, an arrangementof a high-voltage winding 26 as constructed according to the prior art.FIG. 2 is generally representative of a cross-section taken along theline IV-IV of F IG. 1, assuming that the transformer shown in FIG. 1would have been constructed according to the prior art. 1

In FIG. 2, the layers 28 of insulation separate the layers ofconductors. Each conductor layer includes a coil section, such as thecoil section 30. Each coil section includes a plurality of conductorturns, such as the conductor turn 32 of the coil section 30. Theconductor turns are provided by an insulated electrical conductorspirally wound or disposed around the winding axis. Therefore, the coilsection 30 is formed from a conductor which progresses axially from theconductor turn 32' to the conductor turn 32". The coil section 34 isformed from the same or another conductor which progresses axially fromthe conductor turn 36 to the conductor turn 36. The direction ofprogression is basically a function of the method used in winding thecoil section, and is not necessarily a requirement for the properoperation of the winding. However, all of the conductors must be woundtangentially, with respect to the winding axis, in the same direction topermit'the proper phase relationship between interconnected coilsections.

One of the major disadvantages of the winding arrangement shown in FIG.2 is that relatively large axial stresses may develop when the windingsare subjected to high currents, such as exist during a short-circuitcondition. The magnitude of the stresses depends on the position of thetap changer, since certain tap changing positions produce displacedelectrical winding centers between the high-voltage and low-voltagewindings.

Normally, the low-voltage winding is divided into two portions which arecoaxially located on each side of the high-voltage winding. Since thelow-voltage winding is not tapped, the electrical center thereof isfixed at or near the mechanical center of the windings as indicated bythe line 38. When the high-voltage winding 26 is tapped below maximumrating, such as provided schematically by the interconnection 40, theelectrical center of the high-voltage winding 26 is shifted to aposition below the line 38.

FIG. 3 is a force diagram of a transformer wherein the electrical center48 of the high-voltage winding 26 is below the electrical centers 50 and52 of the lowvoltage winding portions 54 and 56, respectively. Theresultant forces generated by the short-circuit stresses in the windingsare represented by the vectors 58 and 60 which act through theelectrical centers. The component vectors indicate that the verticalcomponent forces add together. The physical result is that thehighvoltage winding 26 tends to telescope, or move axially, with respectto the low-voltage winding portions 54 and 56. This is undesirable sincestrong supporting structures must be used to maintain winding integrityduring high stress conditions.

FIG. 4 is generally representative of a partial crosssection of aportion of a high-voltage winding 64 taken along the line IVIV ofFIG. 1. The conductor layers which are not tapped, such as the conductorlayer 66, are constructed substantially similar to the untappedconductor layers illustrated in FlG. 2. The conductor layer 68 containsthe coil sections 74 and 76, and the conductor layer 70 contains thecoil sections 80 and 82. The conductor turns of the coil section 74 areformed by successive turns of an electrical conductor which progressesin the same tangential direction as the conductors which form the otherconductor turns of the winding 64. All adjacent conductor turns areformed by an additional turn of the same conductor with the exception ofthe conductor turns 86 and 88, and the conductor turns 90 and 92. Theinsulation 94 is positioned between these conductor turns since morethan a single turn of voltage exists between these conductor turns.

The conductor turn 86 is substantially noninductively connected to theconductor turn 96 by the interconnection 98. The primary purpose of theinterconnection 98 is to provide direct electrical contact between theconductor turns 86 and 96 without inducing a significant voltage betweenthem, such as would be the case if the conductor turns were electricallyconnected by one or more turns of the conductor. In other words, theconductor turns 86 and 88 are not substantially non-inductivelyconnected to each other since their electrical connection traverses thecoil section 74. A similar connection is provided to the conductor turns90 and 100 by the interconnection 102.

The coil sections 76 and 82 contain the same number of conductor turns.Similarly, the coil sections 74 and 80 contain the same number ofconductors turns. The coil sections 74 and 80 each contain three timesthe number of conductor turns as in the coil sections 76 and 82,respectively. The coil section 80 contains approximately two times thenumber of conductor turns above the tap lead 78 as below the tap lead78. Similarly, the coil section 74 contains approximately two timesthe-number of conductor turns above the tap lead 72 as below the taplead 72. Although the tapped coil sections 74 and 80 are continuouslywound with the same conductor, it is within the contemplation of thisinvention that separate portions of the coil sections could be woundseparately and connected together to provide the same electricalcharacteristics.

More than two coil sections per conductor layer may be used within thecontemplation of this invention to obtain more tap positions as long asthe coil sections have taps which are symmetrical about the windingcenter 104. In addition, the number of conductor turns may be changed,without destroying conductor turn symmetry, to provide different tappingincrements.

Tap leads are connected to the tap terminals A, B, C, D, E and F whichwould normally be a part of the tap changer 22. Depending on which tapterminals are interconnected by the tap changer, certain coil sectionsare electrically removed from the high voltage winding 64. However, withthe proper interconnections, the tap changing can be accomplishedwithout disturbing the symmetry of the high-voltage winding 64, thus itselectrical center remains in line with the electrical centers of thelow-voltage winding portions.

FIG. 5 is a force diagram applicable to the transformer shown in FIG. 1and constructed according to this invention, as shown in FIG. 4. Thediagram is applicable to all tap positions. Since the electrical center108 of the high-voltage winding 64 is in line with the electricalcenters 110 and 112 of the low-voltage winding portions 114 and 116,respectively, the resultant forces do not contain any verticalcomponents. Thus, there is not any tendency for the coils to telescopeunder high stress conditions.

FIG. 6 illustrates how the electrical symmetry is maintained with thevarious tap positions. By referring to both FIGS. 4 and 6, itcan be seenthat when the tap terminals C and D are connected to each other by thetap changer, every coil section of the conductor layers 68 and isincluded in the high-voltage winding. Thus, the symmetry about thecenter line 104 is maintained and the maximum rating exists for thehighvoltage winding 64.

The tap terminals C and E are connected together to lower the voltagerating by one tapping increment, or the voltage developed across thecoil section which is located above the position where the tap lead 78is attached. Since an equal portion of the coil section 80 is tapped outon both sides of the center line 104, the symmetry is maintained.

The tap terminals B and E are connected together to lower the voltagefrom the full rating by two tapping increments. In this position, theupper portions of the coil sections 74 and 80 are tapped out or removedfrom the circuit. However, the symmetry is still maintained. The tapterminals B and F are connected together to lower the voltage from thefull rating by three tapping increments. In this position, the coilsections 80 and 82 and the upper portion of coil section 74 are removedfrom the circuit without changing the symmetry of the winding. The tapterminals A and F are connected together to lower the voltage from thefull rating by four tapping increments. All of the coil sections in thetapped contion without changing the symmetry.

The novel arrangement disclosed herein permits tap changing in equalincrements without changing the electrical symmetry of the tappedwinding. Since numerous changes may be made in the above-describedapparatus, and since different embodiments of the invention may be madewithout departing from the spirit thereof, it is intended that all ofthe matter contained in the foregoing description, or shown in theaccompanying drawing, shall be interpreted as illustrative rather thanlimiting.

I claim as my invention:

1. A tapped transformer comprising:

a magnetic core;

a first winding inductively coupled to said magnetic core;

a tapped second winding having a plurality of coil sections which areinductively coupled to said magnetic core;

a first of said coil sections having a first tap lead connected to thefirst end thereof and a second tap lead connected at a position locatedbetween the first and second ends thereof;

a second of said coil sections being disposed at the same radialposition as said first coil section, with the first end of said secondcoil section being substantially non-inductively connected to'the end ofsaid first coil section which is located opposite to the end which isadjacent to said second coil section;

a third of said coil sections being disposed at a different radialposition than said first coil section, with a third tap lead connectedto the first end thereof and a fourth tap lead connected at a positionlocated between the first and second ends thereof; and

a fourth of said coil sections being disposed at the same radialposition as said third coil section, with the first end of said fourthcoil section being substantially non-inductively connected to the end ofsaid third coil section which is located opposite to the end which isadjacent to said fourth coil section.

2. The tapped transformer of claim 1 wherein the second tap lead isconnected to the first coil section at a position which providestwo-thirds of the total turns of the first coil section between the tapposition and the first end of the first coil section, and one-third ofthe total turns of the first coil section between the tap position andthe second end of the first coil section.

3. The tapped transformer of claim 1 wherein the fourth tap lead isconnected to the third coil section at a position which providestwo-thirds of the total turns of the third coil section between the tapposition and the first end of the third coil section, and one-third ofthe total turns of the third coil section between the tap position andthe second end of the third coil section.

4. The tapped transformer of claim 1 wherein the first end of the secondcoil section is positioned adjacent to the first end of the first coilsection, and the first end of the fourth coil section is positionedadjacent to the first end of the third coil section.

5. The tapped transformer of claim 1 wherein the first and third coilsections are disposed substantially at the same axial position, and thesecond and fourth coil sections are disposed substantially at the sameaxial position.

6. The tapped transformer of claim 1 wherein the conductors which formthe first, second, third and fourth coil sections all spiral around thewinding axis in the same tangential direction.

7. A tapped transformer comprising:

a magnetic core;

a first winding inductively coupled to said magnetic core;

a tapped second winding having a plurality of coil sections which areinductively coupled to said magnetic core;

a first of said coil sections having a predetermined number of turns,with a first tap lead connected to the first end of said first coilsection, a second tap lead connected to said first coil section at aposition which provides two-thirds of the predetermined number of turnsbetween the second tap lead and the first end of said first coilsection;

a .second of said coil sections having one-third the predeterminednumber of turns with a fifth tap lead connected to the second end ofsaid second coil section, the first end of said second coil sectionbeing substantially non-inductively connected to the second end of saidfirst coil section, the first and second coil sections being disposed atthe same radial position with the first end of said second coil sectionlocated adjacent to the first end of said first coil section;

a third of said coil sections being disposed at a different radialposition and at the same axial position as said first coil section, saidthird coil section having said predetermined number of turns with athird tap lead connected to the first end of said third coil section, afourth tap lead connected to the third coil section at a position whichprovides two-thirds of said predetermined number of turns between thefourth tap lead and the first end of said third coil section;

a fourth of said coil sections having one-third said predeterminednumber of turns with a sixth tap lead connected to the second end ofsaid fourth coil section, the first end of said fourth coil sectionbeing substantially non-inductively connected to the second end of saidthird coil section, said third and fourth coil sections being disposedat the same radial position with the first end of said fourth coilsection located adjacent to the first end of said third coil section;and

the conductors which form said first, second, third andfourth coilsections all being disposed to spiral around the winding axis in thesame tangential direction.

1. A tapped transformer comprising: a magnetic core; a first windinginductively coupled to said magnetic core; a tapped second windinghaving a plurality of coil sections which are inductively coupled tosaid magnetic core; a first of said coil sections having a first taplead connected to the first end thereof and a second tap lead connectedat a position located between the first and second ends thereof; asecond of said coil sections being disposed at the same radial positionas said first coil section, with the first end of said second coilsection being substantially non-inductively connected to the end of saidfirst coil section which is located opposite to the end which isadjacent to said second coil section; a third of said coil sectionsbeing diSposed at a different radial position than said first coilsection, with a third tap lead connected to the first end thereof and afourth tap lead connected at a position located between the first andsecond ends thereof; and a fourth of said coil sections being disposedat the same radial position as said third coil section, with the firstend of said fourth coil section being substantially non-inductivelyconnected to the end of said third coil section which is locatedopposite to the end which is adjacent to said fourth coil section. 2.The tapped transformer of claim 1 wherein the second tap lead isconnected to the first coil section at a position which providestwo-thirds of the total turns of the first coil section between the tapposition and the first end of the first coil section, and one-third ofthe total turns of the first coil section between the tap position andthe second end of the first coil section.
 3. The tapped transformer ofclaim 1 wherein the fourth tap lead is connected to the third coilsection at a position which provides two-thirds of the total turns ofthe third coil section between the tap position and the first end of thethird coil section, and one-third of the total turns of the third coilsection between the tap position and the second end of the third coilsection.
 4. The tapped transformer of claim 1 wherein the first end ofthe second coil section is positioned adjacent to the first end of thefirst coil section, and the first end of the fourth coil section ispositioned adjacent to the first end of the third coil section.
 5. Thetapped transformer of claim 1 wherein the first and third coil sectionsare disposed substantially at the same axial position, and the secondand fourth coil sections are disposed substantially at the same axialposition.
 6. The tapped transformer of claim 1 wherein the conductorswhich form the first, second, third and fourth coil sections all spiralaround the winding axis in the same tangential direction.
 7. A tappedtransformer comprising: a magnetic core; a first winding inductivelycoupled to said magnetic core; a tapped second winding having aplurality of coil sections which are inductively coupled to saidmagnetic core; a first of said coil sections having a predeterminednumber of turns, with a first tap lead connected to the first end ofsaid first coil section, a second tap lead connected to said first coilsection at a position which provides two-thirds of the predeterminednumber of turns between the second tap lead and the first end of saidfirst coil section; a second of said coil sections having one-third thepredetermined number of turns with a fifth tap lead connected to thesecond end of said second coil section, the first end of said secondcoil section being substantially non-inductively connected to the secondend of said first coil section, the first and second coil sections beingdisposed at the same radial position with the first end of said secondcoil section located adjacent to the first end of said first coilsection; a third of said coil sections being disposed at a differentradial position and at the same axial position as said first coilsection, said third coil section having said predetermined number ofturns with a third tap lead connected to the first end of said thirdcoil section, a fourth tap lead connected to the third coil section at aposition which provides two-thirds of said predetermined number of turnsbetween the fourth tap lead and the first end of said third coilsection; a fourth of said coil sections having one-third saidpredetermined number of turns with a sixth tap lead connected to thesecond end of said fourth coil section, the first end of said fourthcoil section being substantially non-inductively connected to the secondend of said third coil section, said third and fourth coil sectionsbeing disposed at the same radial position with the first end of saidfourth coil section located adjacent to The first end of said third coilsection; and the conductors which form said first, second, third andfourth coil sections all being disposed to spiral around the windingaxis in the same tangential direction.